"When our time comes, we will make no excuses for the terror"

The actual reason, as said by Hamas's own press releases, is that they do not consider settlers to be civilians. Israel is an occupation of their land, and all occupiers are legitimate targets.

It is almost definitely not Reddit. Reddit fanboys, maybe. But DDoS attacks are highly illegal and not completely untraceable. If it was tied back to Reddit in any way, it would be a lawsuit waiting to happen and horrible PR.